1. Field of the Invention
This invention relates to a novolak aralkyl resin having both structural units of low molecular weight novolak units and aralkyl group units together and a preparation process thereof, and composition containing the resin.
More in particular, it relates to a novolak aralkyl resin which is heightened its molecular weight while increasing the repeating structural units of low molecular weight novolak units and aralkyl group units by suppressing decomposition of low molecular weight novolak, makes its curing reaction with hexamethylenetetramine or the like progress uniformly and rapidly, provides excellent heat resistance to its cured products and suitable to application uses such as friction materials, sliding materials, molding materials and encapsulating materials, a preparation process thereof, a novolak aralkyl resin composition causing rapid curing reaction and providing excellent heat resistance to its cured products which is suitable to friction materials such as disk brake pads, brake linings and clutch facings for braking automobiles, railway vehicles and various industrial machines, to binders for molding materials such as electric and electronic equipment parts, communication equipment parts and mechanical parts and to sliding materials and so on.
In this invention, novolak means, resins having a repeating structure of phenol nuclear units and methylene group units obtained by reacting, for example, phenol and formaldehyde in the presence of an acidic catalyst. They are referred to bi-nuclear novolak, tri-nuclear novolak and the like depending on the number of phenol nuclear units. Further, low molecular weight novolak is a collective term for novolaks up to about penta-nuclear novolak.
2. Related Art Statement
A phenolic resin as a reaction product of a phenol compound and an aralkyl compound such as p-xylylene glycol dimethyl ether is generally referred to as a xylok resin which is a phenol aralkyl resin having a repeating structure of phenol nuclei and aralkyl group as described, for example, in Japanese Patent Publication No. 15111/1972 and Japanese Patent Publication No. 14280/1977. The phenol aralkyl resin has excellent characteristics for heat resistance, soft and flexibility and hygroscopic resistance compared with novolak type phenol resin and has been generally used for applications such as friction materials, molding materials and encapsulating materials. However, in the field where it is used by being cured with hexamethylenetetramine or the like, since the ratio of the phenol nuclei is small in the resin, the curing reaction is slow for which improvement has been desired.
For compensating the drawback, Japanese Patent Laid-Open No. 142324/1992 proposes a modified phenol aralkyl resin obtained by reacting phenols, an aralkyl compound and formaldehyde in the presence of an acidic catalyst at 100 to 150xc2x0 C. Further, Japanese Patent Laid-Open No. 173834/1992 discloses a phenolic resin of using phenol and novolak resin together, which is reacted with p-xylylene glycol dimethyl ether. However, since phenol is used as the raw material in the both resins described above, they contain a phenol aralkyl resin portion causing slow curing reaction, their curing rate are insufficient. In addition, though a novolak resin portion is present during reaction in the method described in both of the publications, when the novolak resin has high molecular weight, it further is heightened molecular weight remarkably or gelled in the subsequent reaction, so that the amount of the aralkyl compound to be introduced can not be increased and, as a result, the amount of unreacted novolak resin increases to bring about a problem of lowering the heat resistance. Further, since the amount of the acidic catalyst used is large, it involves a problem that the catalyst remaining in the resin causes decomposing reaction, failing to obtain a resin of stable property.
In addition, there can be mentioned a method of increasing the velocity of curing reaction by mixing a novolak type phenol resin with the phenol aralkyl resin, but since the novolak type phenol resin reacts preferentially to hexamine or the like, this results in a problem of unevenness in the curing.
When novolak, particularly, a low molecular weight novolak and an acidic catalyst are brought into contact, it causes decomposition and re-bonding to form phenol and high molecular weight novolak. Accordingly, decomposition and re-bonding reaction are also caused in a case of reacting the low molecular weight novolak with the aralkyl compound in the presence of the acidic catalyst, so that it leads to a problem that a stable reaction product containing many repeating structural units of low molecular weight novolak and aralkyl groups can not be obtained.
This invention intends to provide a novolak aralkyl resin having both structures of bi-nuclear novolak units and aralkyl group units together and being capable of conducting the curing reaction with hexamethylenetetramine or the like rapidly and uniformly while maintaining the excellent heat resistance inherent to the phenol aralkyl resin, and a preparation process thereof, and a composition containing the novolak aralkyl resin described above.
An aimed novolak aralkyl resin can be obtained by increasing the repeating structural units of bi-nuclear novolak units and aralkyl group units and by increasing the molecular weight of the resin. In this case, it is important to suppress the decomposing reaction of the low molecular weight novolak. Novolak causes decomposition and re-bonding by being heated in the presence of an acidic substance to form phenol and high molecular weight novolak. For example, when a bi-nuclear novolak as a typical example of the low molecular weight novolak is heated for about one hour in the presence of the same acidic catalyst and at the same temperature as upon reaction with the aralkyl compound, about 8 mol % phenol is formed, and twice molar amount of bi-nuclear novolak is lost and tri-or higher poly-nuclear novolak is formed. On the other hand, when the reaction velocity with the aralkyl compounds is compared between the low molecular weight novolak and phenol, the reaction velocity is higher for the low molecular weight novolak. For example, when the mixture of bi-nuclear novolak and phenol containing each at an equal molar amount is reacted with the aralkyl compound, it results in more unreacted component for phenol.
Accordingly, even if the bi-nuclear novolak and the aralkyl compound are charged each by a predetermined amount for reaction into the reaction system, when the decomposition of the bi-nuclear novolak proceeds, reaction is taken place for three ingredients of novolak, phenol and aralkyl compound and the properties of the resultant resin is deviated from desired values depending on the extent of the decomposition. Further, after the completion of the reaction, in a case where the decomposition proceeds while liberating a phenol, the resultant resin becomes instable because of bonding of disconnected active residues with other molecules of bi-nuclear novolak or the like. Consequently, the molecular weight of the resultant resin becomes higher.
From the foregoings, as a means for obtaining a resin containing many repeating structural units of low molecular weight novolak and aralkyl groups, it is important to suppress the decomposing reaction of the low molecular weight novolak and to avoid the formation of the free phenol as an index of the decomposition of raw materials such as low molecular weight novolak as less as possible.
The present inventors have made an earnest study on the basis of the knowledge as described above, as a result, have found that the decomposition of the low molecular weight novolak during the reaction can be suppressed to obtain an aimed resin upon reacting low molecular weight novolak containing a bi-nuclear novolak at a specified amount or more and an aralkyl compound in the presence of an acidic catalyst by suppressing contact of the acidic catalyst only with the low molecular weight novolak as the raw material, restricting the amount of the catalyst to a required minimum level as less as possible and, further, neutralizing the acidic catalyst after the completion of the reaction, and have accomplished this invention.
That is, the first feature of this invention resides in a preparation process for a novolak aralkyl resin by reacting 0.4 to 0.8 mol of an aralkyl compound based on one mol of a low molecular weight novolak containing 90% by weight or more of a bi-nuclear novolak in presence of an acidic catalyst, wherein comprising at first melting the low molecular weight novolak and heating it up to a reaction temperature, then adding 0.001 to 0.05% by weight of the acidic catalyst based on the total amount of the low molecular weight novolak and the aralkyl compound, then continuously adding the aralkyl compound for reaction, neutralizing the residual acidic catalyst after the completion of the reaction.
As a preferred embodiment for the preparation process for the novolak aralkyl resin described above, there can be mentioned a method of using p-xylylene glycol dimethyl ether as an aralkyl compound, a method of practicing the reaction at a reaction temperature within a range of 130 to 160xc2x0 C., a method of using hydroxides of calcium, barium, magnesium or a mixture of such metals as a neutralizing agent.
The second feature of this invention resides in a novolak aralkyl resin obtained by the preparation process described above. The resin is a novolak aralkyl resin represented by the general formula (1):
xe2x80x83xe2x80x83(1)

wherein m is an integer of 1 to 4 and n is an integer of 1 to 10,000, in which the ratio of the low molecular weight novolak unit with m being 1 based on low molecular weight novolak unit with m being 1 to 4 in the general formula (1) described above is at least 80% by weight, the hydroxyl equivalent is 120 to 145 g/eq, and the content of the free phenol is 2% by weight or less.
The third feature of this invention resides in a novolak aralkyl resin composition containing 80 to 95% by weight of the novolak aralkyl resin described above and 5 to 20% by weight of the hexamethylenetetramine. The resin composition has such characteristics that the 90% curing time at 150xc2x0 C. is 7 to 12 minutes and the weight retention ratio after storage at 300xc2x0 C. for 240 hours is 70% or more.
A molding base material or a solvent may be added to the resin composition depending on the application use. When the molding base material is added, 80 to 95% by weight of at least one molding base material selected from the group consisting of reinforcing fibers, lubricants and fillers is added based on 5 to 20% by weight of the resin composition. Further, when the solvent is added, 30 to 70% by weight of an organic solvent is added based on 30 to 70% by weight of the resin composition. A preferred solvent can include methanol, ethanol, methyl ethyl ketone, butyl cellosolve, butyl carbitol or a mixture of them.
The fourth feature of this invention is a novolak aralkyl resin composition containing 10 to 75% by weight of the novolak aralkyl resin described above, 25 to 90% by weight of an epoxy resin and 0.01 to 5% by weight of a curing catalyst based on the total amount of both of the resins.
In a case where the resin composition is used, for example, as a encapsulating material, 100 to 190 parts by weight of an organic filler, inorganic filler or a mixture thereof is added based on 100 parts by weight of the resin composition.
According to this invention, decomposition of the low molecular weight novolak can be suppressed in the entire preparation steps from the charging of the raw materials to the formation of a novolak aralkyl resin, consequently, the novolak aralkyl resin with a high ratio of BPF unit based on the low molecular weight novolak unit with m being 1 to 4 in the general formula (1) can be obtained. The novolak aralkyl resin contains many repeating structures of BPF units and aralkyl group units. Since the novolak aralkyl resin according to this invention has the same heat resistance substantially as existent phenol aralkyl resins and conducts the curing reaction with hexamethylenetetramine or the like uniformly and rapidly. Therefore, a resin composition containing the novolak aralkyl resin according to this invention has both excellent heat resistance and moldability. Accordingly, the novolak aralkyl resin and the composition containing the resin of this invention can be used suitably as binders for molding materials such as friction materials for disk brake pads, brake linings and clutch facings used for braking automobiles, railway vehicles and various industrial machine, electric and electronic equipment parts, communication equipment parts and machine parts, sliding materials, semiconductor encapsulating materials, lamination materials, coatings and adhesives.
The ratio of the low molecular weight novolak unit with m being 1 based on the low molecular weight novolak unit with m being 1 to 4 in the general formula (1) described above in the novolak aralkyl resin according to this invention means a value determined by a method shown in the example to be described later. In this invention, the low molecular weight novolak unit with m being 1 in the general formula (1) means a bi-nuclear novolak unit.
This invention is to be described in details. At first, a preparation process for a novolak aralkyl resin according to the first feature of this invention is to be described. The outline of the preparation process for the novolak aralkyl resin according to this invention is as follows. The preparation process, which comprises reacting a low molecular weight novolak containing a specified amount or more of a bi-nuclear novolak (hereinafter simply referred to as BPF) with an aralkyl compound in the presence of a relatively small amount of an acidic catalyst and neutralizing the residual catalyst after the completion of the reaction.
Usually, the low molecular weight novolak includes a novolak resin with 65% by weight or more of BPF content obtained by reacting about 6 to 30 moles of phenol with 1 mole of formaldehyde in the presence of an acidic catalyst and then removing unreacted phenol (hereinafter simply referred to as de-phenol product), and a novolak resin with 90% by weight or more of BPF obtained by distillation from the de-phenol product, and a novolak resin with non-distilled tri-nuclear novolak as the main ingredient. The amount of BPF in the low molecular weight novolak gives an effect on the heat resistance of the resultant resin. When the amount of BPF is smaller, the ratio of the BPF unit in the resultant novolak aralkyl resin is lowered and the repeating structure of the BPF units and the aralkyl units is decreased. That is, since the novolak resin portion is increased, the heat resistance of the resultant resin is lowered. Further, it is preferred to use the low molecular weight novolak having stable composition such as the BPF content as the raw material for producing a novolak aralkyl resin having stable qualities. With a view point described above, it is preferred in this invention to use a low molecular weight novolak with the BPF content of 90% by weight or more in the low molecular weight novolak described above.
The aralkyl compound used in this invention, can include, for example, xcex1, xcex1xe2x80x2-dichloro-p-xylene, xcex1, xcex1xe2x80x2-dichloro-o-xylene, xcex1, xcex1xe2x80x2-dichloro-m-xylene, p-xylylene glycol, p-xylylene glycol dimethyl ether (hereinafter simply referred to as PXDM), xcex1, xcex1xe2x80x2-dimethoxy-o-xylene, xcex1, xcex1xe2x80x2-dimethoxy-m-xylene, and xcex1, xcex1xe2x80x2-dimethoxy-p-xylene. A preferred aralkyl compound is PXDM.
As PXDM, a high purity product of 98% by weight or more is preferred. A PXDM obtained industrially usually contains impurities such as xcex1-methoxy-p-xylene, xcex1, xcex1-dimethoxy-p-xylene, xcex1, xcex1, xcex1xe2x80x2-trimethoxy-p-xylene, p-xylylene glycol and p-xylylene glycol monomethyl ether. However, there is no problem when the amount of the impurities is less than 2% by weight.
The amount of the low molecular weight novolak and the aralkyl compound used gives an effect on the curing reaction with hexamethylenetetramine (hereinafter simply referred to as hexamine) or the like and the heat resistance of the resultant resin. For obtaining a resin having excellent heat resistance and curing reactivity, it is preferred to use 0.4 to 0.8 mole of the aralkyl compound based on one mole of the low molecular weight novolak. A further preferred amount of the aralkyl compound is 0.5 to 0.75 mole. When the amount of the aralkyl compound used exceeds 0.8 mole, high molecular weight novolak alalkyl resin increases remarkably or gelation of the resin occurs. Consequently, the aimed resin can not be obtained. On the contrary, when it is less than 0.4 mole, since unreacted low molecular weight novolak increases, the molecular weight of the resin is not increased and the heat resistance of the resin is lowered.
The low molecular weight novolak and the aralkyl compound are reacted in the presence of an acidic catalyst. As Preferable acidic catalyst, for example, zinc chloride, stannic chloride, sodium hydrogen sulfate, sulfuric acid, hydrochloric acid, oxalic acid, monoethyl sulfuric acid, diethyl sulfate, phenol sulfonic acid and p-toluene sulfonic acid are illustrated. Diethyl sulfate is more Preferable. For suppressing the decomposition reaction of the low molecular weight novolak and completing the condensation reaction rapidly, the amount of the acidic catalyst used is preferable within a range from 0.001 to 0.05% by weight based on the total amount of the low molecular weight novolak and the aralkyl compound as using the raw materials. A further preferred range is from 0.005 to 0.02% by weight. When the amount exceeds 0.05% by weight, decomposition of the low molecular weight novolak increases. On the contrary, when the amount is less than 0.001% by weight, the reaction velocity is lowered.
In the presence of the acidic catalyst, change of the properties occurs in the resultant resin even after the aralkyl compound has been consumed thoroughly to complete the reaction. Specifically, unreacted low molecular weight novolak and the novolak resin portion of the resultant resin are decomposed while liberating phenol and re-bonded with them to become high molecular weight resin. Accordingly, for stabilizing the properties of the resin, it is important to conduct neutralization of the residual acidic catalyst after the completion of the reaction.
There is no particular restriction on the neutralizing agent so long as it is basic, hydroxides of alkali metals such as lithium, sodium and potassium or alkaline earth metals such as magnesium, calcium or barium are preferred. Hydroxide of magnesium, calcium or barium is further preferred. Barium hydroxide which gives less effect on the properties of the resin and which neutralization salt to be formed is used as a filler for friction materials is further preferred.
The amount of the neutralizing agent used is preferably from 0.8 to 1.1 equivalent based on the acidic catalyst while depending on the amount of the acidic catalyst used. 1.0 equivalent is most preferred. When it is less than 0.8 equivalent, non-neutralizing acidic catalyst undesirably causes change of the properties of the resin. On the other hand, when it exceeds 1.1 equivalent, the resin is undesirably tinted yellow.
When PXDM is used as the aralkyl compound, it is also useful to use a small amount of methanol upon practicing this process. Though the reaction of the low molecular weight novolak with PXDM by-produces methanol, elevation of the reaction temperature at the initial stage of the reaction can be prevented by previously adding the small amount of methanol prior to the start of the reaction. That is, though heat is generated upon starting of the reaction and the internal temperature rises till the by-produced methanol is saturated in the reaction system, the internal temperature elevation can be prevented by previously adding the saturation amount of methanol, which leads to the effect of preventing decomposition of the low molecular weight novolak. The amount of methanol to be added is preferably 2 to 4% by weight based on the low molecular weight novolak, for example, in a case where the reaction temperature is 130 to 160xc2x0 C.
In addition to the ingredients described above, addition of commercial defoamer silicone is also useful. Particularly, in a case of producing a high molecular weight resin, it can provide an effect of easily withdrawing by-produced methanol from the reaction system to prevent rise of the liquid surface of the reaction product and further shorten the time of operation such as removal of methanol conducted under a reduced pressure. A preferred addition amount of the silicone is 10 to 20 ppm based on the resin formed.
A preferred embodiment of the preparation process according to this invention is as described below. After charging a low molecular weight novolak and a small amount of methanol into a reactor and elevating the temperature up to reaction temperature, an acidic catalyst is added. Then, continuous charging of PXDM is initiated. Charging of PXDM is continued while distilling off the by-produced methanol and, when charging of a predetermined amount of the PXDM is completed, aging reaction is conducted to complete the reaction. Then, the catalyst is neutralized and a trace amount of methanol and water dissolved therein are removed under a reduced pressure.
The reaction temperature is preferable within a range from 120 to 200xc2x0 C. A more preferred range is from 130 to 160xc2x0 C. When it is lower than 120xc2x0 C., the reaction velocity is extremely slow. On the other hand, when it exceeds 200xc2x0 C., decomposition of the low molecular weight novolak used as raw materials and the reaction product increases. Further, in a case of using PXDM as the aralkyl compound, a portion of PXDM is extracted together with by-produced methanol out of the reaction system.
Even though the low molecular weight novolak with a high BPF content is used as raw material, when the decomposition of the low molecular weight novolak during reaction increases, the ratio of the BPF unit in the resultant novolak aralkyl resin is lowered, and it results in decreasing of the repeating structure of the BPF units and the aralkyl units. That is, since the novolak resin portion increases, the heat resistance of the resultant novolak aralkyl resin is lowered. Accordingly, it is necessary to react by a method of suppressing the decomposition of the low molecular weight novolak.
The decomposition of the low molecular weight novolak takes place most remarkably in the step from the instance the catalyst is added to the start of the charging of the aralkyl compound. Therefore, it leads to the prevention of decomposition to shorten the time of contact only between the low molecular weight novolak and the catalyst. Accordingly, it is preferred to start the charging of the aralkyl compound immediately after addition of the catalyst. Further, for obtaining a resin having stable characteristics, it is preferred to shorten the time of contact only between the low molecular weight novolak and the catalyst as less as possible. The shorter contact time is more preferred for reducing the deviation in the characteristics of the resin, it is preferably less than 30 minutes, particular preferably, less than 15 minutes. If necessary, a portion of the aralkyl compound to be charged may be present in the reaction system before addition of the catalyst.
The method of suppressing the decomposition of the low molecular weight novolak can include a method of charging the whole raw materials at the same time and a method of adding the catalyst continuously. Specifically, the former is a method of charging the low molecular weight novolak, the aralkyl compound and the catalyst at the same time, then initiating the reaction, while the latter is a method of charging the low molecular weight novolak and the aralkyl compound at the same time, then continuously adding the catalyst such as hydrochloric acid to conduct reaction. Each of the methods can suppress the decomposition of the low molecular weight novolak and suppress the formation of phenol. However, they involve such problems that the control for the reaction velocity or the molecular weight is difficult, the reactor efficiency is poor, high molecular weight compound insoluble to the solvent is formed on the reactor wall and a great amount of catalyst is required.
As a result of considering them, the present inventors have reached a preparation process of this invention of at first charging the low molecular weight novolak, elevating the temperature up to the reaction temperature, adding the acidic catalyst and then instantly starting the continuous charging of the aralkyl compound.
The continuous charging time of the aralkyl compound depends on the molar ratio of the raw materials, the reaction temperature and the scale of the reaction. It is usually from 30 minutes to 10 hours at the preferred reaction temperature described above. It is preferably, 1 to 6 hours. When it is less than 30 minutes, unreacted aralkyl compound increases to require a long time for the aging reaction substantially. On the contrary, if it exceeds 10 hours, decomposition of the low molecular weight novolak increases and the productivity is lowered.
The aging reaction is conducted till the completion of the reaction. Completion of the reaction means that the unreacted aralkyl compound is no more present in the reaction system. Though the aging time required differs depending on the amount of the catalyst, the reaction temperature and the charging time of the aralkyl compound, it is about 30 minutes to 5 hours. For example, it requires about three hours in a case where PXDM is used for the aralkyl compound, the catalyst is used by 0.01% by weight based on the total amount of the low molecular weight novolak and the PXDM, the reaction temperature is set at 145xc2x0 C. and PXDM is charged for three hours for reaction.
Since decomposition proceeds also even after the completion of the reaction by the contact of the reaction product and the unreacted low molecular weight novolak with the acidic catalyst, to give an effect on the properties of the resultant resin, it is important to instantly neutralize the residual catalyst after the completion of the aging reaction. Particularly, since the pressure reduction step and the discharging operation are conducted at a high temperature for a long time, neutralization of the residual catalyst gives a significant effect for the stabilization of the properties of the resin. The neutralizing agent may be added either in the form a solid as it is or in the form of a solution. Since the amount of it to be used is small amount, it is effective to conduct neutralization in the form of an aqueous solution for rapid neutralization. Since the reaction liquid after the neutralization contains a small amount of methanol and water containing neutralization agent dissolved therein, they are distilled off under a reduced pressure.
In the preparation process according to this invention, since a method of charging the low molecular weight novolak, elevating the temperature up to the reaction temperature, adding a specified amount of the acidic catalyst and then continuously charging the aralkyl compound is adopted, therefore, the decomposition reaction of the low molecular weight novolak and the reaction product that occurs from the charging of the raw material to the completion of the condensation reaction can be suppressed. Accordingly, the novolak aralkyl resin obtained by the preparation process according to this invention contains 2% by weight or less of free phenol formed mainly by the decomposition of the low molecular weight novolak. The free phenol optionally can be reduced further by being removed strictly under a reduced pressure. The content of the free phenol is most preferably 0% by weight. The content of the free phenol of 2% by weight or less formed by the decomposition of the low molecular weight novolak means that the decomposition amount of the low molecular weight novolak is 5 mol % or less. When the decomposition is kept at such an extent or less, a resin having desired properties can be obtained by using the low molecular weight novolak and the aralkyl compound at a predetermined ratio.
In the preparation process for the novolak aralkyl resin according to this invention, since the low molecular weight novolak as the raw material is used in excess relative to the aralkyl compound and since the reaction is conducted while suppressing the decomposition of the low molecular weight novolak till the aralkyl compound is no more present in the reaction system, the resultant resin contains 8 to 20% by weight of BPF. Accordingly, the resin with the BPF content of less than 8% by weight with no particular operation for removing BPF shows that this is a resin suffering from remarkable decomposition of the low molecular weight novolak.
The unreacted BPF can also be decreased optionally, for example, by a method of distillation, hot water extraction or steam stripping under the conditions at a temperature of 200 to 250xc2x0 C. and at a pressure of 0.1 to 6.7 kPa. The resin removed with the unreacted BPF has a feature of having only the repeating structure of the low molecular weight novolak units and the aralkyl group units. Further, unreacted BPF can be decomposed and reduced also by continuing heating without neutralization even after the completion of the condensation reaction, but the properties of the resultant resin are remarkably changed undesirably.
The second feature of this invention resides in the novolak aralkyl resin obtained by the preparation process described previously. According to the preparation process for the resin, the decomposition reaction of the low molecular weight novolak and the reaction product that occurs during reaction of the low molecular weight novolak with the aralkyl compound is suppressed. Accordingly, in the novolak aralkyl resin according to the second feature of this invention, the ratio of the low molecular weight novolak with m being 1 based on a low molecular weight novolak unit with m being 1 to 4 in the general formula (1) described above is at least 80% by weight. The novolak aralkyl resin having such a structure is excellent in the heat resistance and has high curing reaction velocity.
The novolak aralkyl resin according to the second feature of this invention produced as described above has a hydroxyl equivalent of 120 to 145 g/eq. It is preferably from 130 to 140 g/eq. When it is less than 120 g/eq, the resin is of low molecular weight with less aralkyl group ratio and many unreacted low molecular weight novolak is contained and heat resistance is low. When it exceeds 145 g/eq, the resin has an extremely high molecular weight and the molding becomes difficult.
The hydroxyl equivalent can be forecast approximately based on the ratio of the low molecular weight novolak and the aralkyl compound used. Even though the decomposition occurs remarkably and much phenol is formed, it is possible to obtain a resin of the preferred range described above unless the phenol is removed out of the reaction system. Further, the resin having similar hydroxyl group equivalent can be obtained also by a method of mixing the phenol aralkyl resin and the novolak resin, a method of using phenol and novolak together to react them with PXDM as described in Japanese Published Unexamined Patent Laid-open No. 173834/1992. However, even though the hydroxyl equivalent is adjusted within the range as specified in this invention by these methods described above, the resin represented by the general formula (1) according to this invention can not be obtained and the curing reaction with hexamine or the like proceeds not uniformly.
According to the preparation process of this invention, decomposition of the low molecular weight novolak during the condensation reaction is suppressed. Accordingly, it is possible to obtain a novolak aralkyl resin having less content of the free phenol as the index of the decomposition, having a high ratio of the BPF unit based on the low molecular weight novolak units with m being 1 to 4 in the general formula (1) described above and containing many repeating structures of the BPF units and the aralkyl group units. Since the novolak aralkyl resin according to this invention conducts curing reaction with hexamine or the like uniformly and rapidly, it is possible to obtain a resin composition suitable to friction materials by mixing with hexamine and molding materials such as reinforcing fiber, lubricant or filler. Further, since the acidic catalyst has been neutralized, a stable modified resin is obtained also in a case of modifying with a silicone rubber. Further, when it is used as a curing agent of an epoxy resin, since the hydroxyl equivalent is small, it provides a merit capable of decreasing the amount to be used.
Then, description is to be made to a novolak aralkyl resin composition according to the third feature of this invention. The novolak aralkyl resin composition according to this invention is produced by adding and mixing a specified amount of hexamine to the novolak aralkyl resin described above. There is no particular restriction on the mixing method and it can include, for example, a method of pulverizing and finely powderizing while mixing by using a pulverizer or the like. The mixing temperature is preferably near the room temperature.
The blending ratio of the hexamine with the novolak aralkyl resin gives an effect, for example, on the curing velocity, curing degree, heat resistance of curing product and the operation circumstance. When the addition amount of the hexamine is insufficient, the curing velocity of the resin is slow, no sufficient curing degree is obtained and it is difficult to obtain a cured product having excellent heat resistance. On the contrary, if the addition amount is excessive, excess hexamine is decomposed to evolve a great amount of ammonia, which is not preferred in view of the operation circumstance. Considering them, it is preferred to mix 80 to 95% by weight of the novolak aralkyl resin and 5 to 20% by weight of hexamine. Further preferably, it is within a range from 87 to 93% by weight of the novolak aralkyl resin and from 7 to 13% by weight of hexamine.
The novolak aralkyl resin composition according to the third feature of this invention requires a short time to reach 90% curing ratio of 7 to 12 minutes when cured at 150xc2x0 C. and the curing velocity is extremely high compared with existent phenol aralkyl resins. Further, the molding product obtained by curing the resin composition at 150xc2x0 C. has a weight retention ratio of 70% or more after storage at 300xc2x0 C. for 240 hours and shows excellent heat resistance much more than the novolak resin and shows the same heat resistance as the phenol aralkyl resin. Accordingly, the novolak aralkyl resin composition according to this invention is a novel resin composition having the excellent heat resistance and high speed molding property together.
When the resin composition according to the third feature of this invention is used, for example, to a friction material, it is preferred to add molding base materials such as reinforcing fibers, lubricants and fillers to the resin composition containing the novolak aralkyl resin and the hexamine as described above. In this case, they are mixed within a range of 5 to 20% by weight of the resin composition containing the novolak aralkyl resin and the hexamine and 80 to 95% by weight of at least one molding base material selected from the group consisting of reinforcing fibers, lubricants and fillers.
The reinforcing fiber in this invention can include inorganic, organic and metal fibers such as glass fibers, carbon fibers, alamide fibers and steel fibers. The lubricant can include, graphite, antimony sulfide and molybdenum sulfide. Further, the filler can include, for example, cashew dust, barium sulfate, calcium carbonate, magnesium carbonate, silica and metal powder. The molding base material may be used singly, or two or more of them may be used as a mixture.
When the resin composition according to this invention is used, for example, to wet friction materials, adhesives or sliding materials, it is preferred to add a solvent to the resin composition containing the resin and the hexamine. In this case, 30 to 70% by weight of the resin composition containing the novolak aralkyl resin and hexamine and 30 to 70% by weight of the solvent are mixed to dissolve the resin composition. A preferred solvent can include at least one solvent selected from the group consisting of methanol, ethanol, methyl ethyl ketone, butyl cellosolve and butyl carbitol.
Then, description is to be made to a novolak aralkyl resin composition according to the fourth feature of this invention. The novolak aralkyl resin composition according to this invention is also prepared by mixing the novolak aralkyl resin described above, an epoxy resin and a curing catalyst. There is no particular restriction on the mixing method and, for example, a method of dissolving each of them in a solvent or a pulverization mixing method. The mixing temperature is preferably near the room temperature. A cured product is obtained by applying a heat treatment to the novolak aralkyl resin composition according to this invention within a temperature range usually from 100 to 250xc2x0 C.
The epoxy resin may be any epoxy resin so long as two or more epoxy groups are contained in one molecule and it can include, for example, bisphenol-A type epoxy resin, bisphenol-F type epoxy resin, phenol novolak type epoxy resin, cresol novolak type epoxy resin, glycidyl ether type epoxy resin such as tetramethyl biphenyl type epoxy resin, cycloaliphatic epoxy resin such as (3xe2x80x2, 4xe2x80x2-epoxy cyclohexyl methyl)-3-4-epoxy cyclohexane carboxylate. Two or more of the epoxy resins may be used together. Typical commercial products of the cresol novolak type epoxy resin can include, for example, EOCN-102S; trade name of product manufactured by Nippon Kayaku Co.
The blending ratio of the epoxy resin to the novolak aralkyl resin gives an effect on the curing velocity, the curing degree, and the heat resistance and hygroscopic resistance of the curing product. When the blending amount of the novolak aralkyl resin is insufficient, the curing velocity of the resin composition is slow, no sufficient curing degree is obtained and it is difficult to obtain a cured product having excellent heat resistance and hygroscopic resistance. Further, when the blending ratio is excessive, the performances described above can not be provided at a good balance, which is also not preferred. In view of the above, it is preferred to mix the novolak aralkyl resin from 10 to 75% by weight and the epoxy resin from 25 to 90% by weight being expressed by weight percentage. Further preferably, the novolak aralkyl resin ranges from 15 to 70% by weight and the epoxy resin ranges from 30 to 85% by weight. Further, the equivalent ratio of the hydroxyl groups in the novolak aralkyl resin and the epoxy groups in the epoxy resin is from 0.5 to 1.5 molar equivalent amount and, preferably, from 0.7 to 1.3 molar equivalent amount of the hydroxyl groups based on 1 molar equivalent amount of the epoxy group and it is preferred to adjust the molar ratio so as to obtain the optimal properties of the curing product.
The curing catalyst can include, for example, organic phosphine compounds such as triphenyl phosphine, imidazole compounds such as 2-ethyl-4-methylimidazole and bi-cyclic nitrogen containing compounds such as 1,8-diazabicyclo (5,4,0) undeca-7-ene. The addition amount of the curing catalyst is 0.01 to 5% by weight, preferably, 0.05 to 1% by weight based on the total weight of the novolak aralkyl resin and the epoxy resin.
An organic filler, an inorganic filler, a mixture thereof or other additives may optionally added to the novolak aralkyl resin composition according to the fourth feature of this invention. It is particularly preferred to use the organic filler, inorganic filler or the mixture thereof for improving the mechanical properties or for reducing the entire cost, a colorant such as carbon black for preventing erroneous operation by light and, further, a mold release agent, a coupling agent, a flame retardant such as antimony trioxide and a flexibilizer such as acrylonitrile, butadiene rubber and silicone oil.
The organic filler, inorganic filler or the mixture thereof to be used can include, for example, powder such as silica, alumina, silicon nitride, silicon carbide, talc, calcium silicate, calcium carbonate, mica, clay and titanium white, and fibers such as glass fibers, carbon fibers and alamide fibers.
The amount of the organic filler, the inorganic filler or the mixture thereof to be used is from 100 to 1900 parts by weight based on 100 parts by weight of the novolak aralkyl resin composition. In view of the hygroscopic resistance and the mechanical strength, it is preferably from 250 to 1900 parts by weight and, more preferably, from 550 to 1900 parts by weight. The novolak aralkyl resin composition according to the fourth feature of this invention can provide a cured product having excellent heat resistance and hygroscopic resistance. Accordingly, it is used suitably, for example, to semiconductor encapsulating materials, lamination materials, coatings, adhesives and molding materials.